1.FIELD OF THE INVENTION
The present invention relates to transfer-type plasma torches and, more particularly, to the electrode structure in the plasma generating portion. Transfer-type plasma torches which the present invention is concerned with may be used to heat objects, for example, to heat molten steel at a certain stage of being supplied from a converter to a continuous casting mold.
2. DESCRIPTION OF THE PRIOR ART
Induction heating or heating by means of a plasma torch is effected to heat an object such as molten steel. There are two types of plasma torches, one being a transfer type, and the other being a non-transfer type. In a plasma torch of the transfer type, an object to be heated is set as the anode, and electric discharge is effected between the cathode of the plasma torch and the object to be heated. In a plasma torch of the non-transfer type, electric discharge is effected between the cathode and the anode of the plasma torch, a processing gas is supplied to the space between these electrodes, and the gas passed through the space between the cathode and the anode is applied to the object to be heated.
A processing gas (preferably an inert gas) such as N.sub.2 or Ar is also used in the case of transfer type plasma torches for the purpose of shielding the electrodes from the ambient atmosphere. However, non-transfer type plasma torches consume a much larger amount of processing gas. Because of this large amount of consumption of a processing gas, non-transfer type plasma torches involve high operation cost.
FIGS. 7, 8, and 9a to 9c show a conventional transfer-type plasma torch disclosed in Japanese Patent Unexamined Publication No. 54-136193. FIG. 7 is a longitudinal section of the end portion of the plasma torch, FIG. 8 is a view of an electric circuit including the plasma torch, FIGS. 9a, 9b, and 9c are views showing in detail different arrangements which may be provided at the tip portion of the cathode of the plasma torch.
The conventional plasma torch has an auxiliary electrode 19 in the center, a cylindrical cathode 17 around the auxiliary electrode 19, and a cylindrical nozzle 18 around the cathode 17.
A processing gas is caused to flow both into the gap between the auxiliary electrode 19 and the cathode 17 and into the gap between the cathode 17 and the nozzle 18. The flow rates of the processing gas are set in such a manner that the ratio between the flow in the gap between the auxiliary electrode 19 and the cathode 17 and that in the gap between the cathode 17 and the nozzle 18 is 1:5 to 8. Thus, the flow of processing gas in the gap between the cathode 17 and the nozzle 18 corresponds to the majority of the entire flow.
With the conventional plasma torch, plasma is generated in the following manner. First, the processing gas is introduced. At the time of ignition, a high voltage at a high frequency is applied to the gap between the auxiliary electrode 19 and the cathode 17, thereby causing electric discharge in this gap. Thereafter, a DC voltage is applied by using the cathode 17 as the minus electrode and the auxiliary electrode 19 as the plus electrode, thereby generating a pilot arc. When the generation of the pilot arc has been achieved in this way, the application of the high-frequency voltage for the ignition is terminated. Subsequently, a DC voltage is applied by using the cathode 17 as the minus electrode and an object 20 to be heated as the plus electrode, thereby generating a main arc therebetween. The object 20 is heated by the main arc.
The application of DC voltage to the cathode 17 and the auxiliary electrode 19 is continued also during the time in which the main arc keeps generating, so that the pilot arc is always generated during that time.
The pilot arc serves, together with the introduction of a large amount of cool processing gas into the gap between the cathode 17 and the nozzle 18, to prevent any electric discharge from the cathode 17 to the nozzle 18 and, hence, to prevent any damage to the nozzle 18.
As regards the configuration of the cathode 17, in order to ensure that the plasma arc generating region is stably formed, the central passage of the cathode 17 should as much as possible be provided with an enlarged portion which has its length set at a dimension 0.1 to 0.2 times the outer diameter D.sub.1 of the cathode 17, and has its diameter D.sub.1 in the vicinity of the surface of the cathode 17 set at a dimension 2 to 5 times the diameter d.sub.1 of the adjacent portion of the central passage. This enlarged portion of the central passage may either be shaped like a frustum of a cone or a cylinder. If this arrangement is provided, it is possible to ensure, in addition to stable formation of the plasma arc generating region, dispersion of the plasma arc generating region over the entire area of the enlarged portion of the central passage, this dispersion enabling a reduction in the current density on the electrode surface.
The electric circuit shown in FIG. 8 includes a power source 21 connected to the cathode 17 and the auxiliary electrode 19, a main arc power source 23 for generating a main arc in the gap between the cathode 17 and the object 20 to be heated, and a high frequency generator 22.
The above-described conventional transfer-type plasma torch, however, involves the following disadvantages. In order to ensure stable formation of the plasma arc generating region as well as dispersion of the plasma arc generating region over the entire area of the enlarged portion of the central passage and, hence, a reduction in the current density on the electrode surface, a certain number of charged particles which is large enough to compensate for the space charge adjacent to the effective surface of the electrode must be always generated and supplied by the pilot arc. Furthermore, in order to maintain this space charge stably in the vicinity of the electrode, and simultaneously prevent any damage to the edge portion at the tip of the cathode due to displacement of the main arc to this portion, any reduction in the heating efficiency due to failure of the proper convergence of the plasma arc, and any damage to the nozzle due to electric discharge from the cathode to the nozzle, it is necessary to supply a large amount of cool processing gas into the gap between the cathode 17 and the nozzle 18.
With the arrangement of the conventional plasma torch, therefore, the supply of a large amount of processing gas to the nozzle and into the gap between the nozzle and the cathode is essential, as mentioned before.
Thus, the provision of a nozzle, which has conventionally been adopted, involves the following drawbacks:
(1) The outer diameter of the plasma torch becomes three times or more that of the cathode, causing a great increase in weight, and also an increase in the space required for installation.
(2) Since a large amount of processing gas has to be consumed, this is disadvantageous in terms of economy.
(3) Since the gas has to be supplied in two lines while nozzle cooling water is also necessary, the structure of the torch and the systems for supplying the gas and the water are inevitably complicated.
Furthermore, with the conventional arrangement, the pilot arc must be always generated during operation.